Improve constructor resolution in variant_caster

Currently, `py::int_(1).cast<variant<double, int>>()` fills the `double`
slot of the variant. This commit switches the loader to a 2-pass scheme
in order to correctly fill the `int` slot.

include/pybind11/stl.h

@@ -315,6 +315,12 @@ struct variant_caster<V<Ts...>> {
     bool load_alternative(handle, bool, type_list<>) { return false; }
 
     bool load(handle src, bool convert) {
+        // Do a first pass without conversions to improve constructor resolution.
+        // E.g. `py::int_(1).cast<variant<double, int>>()` needs to fill the `int`
+        // slot of the variant. Without two-pass loading `double` would be filled
+        // because it appears first and a conversion is possible.
+        if (convert && load_alternative(src, false, type_list<Ts...>{}))
+            return true;
         return load_alternative(src, convert, type_list<Ts...>{});
     }
 

tests/test_python_types.cpp

@@ -366,6 +366,10 @@ test_initializer python_types([](py::module &m) {
         return std::visit(visitor(), v);
     });
 
+    m.def("load_variant_2pass", [](std::variant<double, int> v) {
+        return std::visit(visitor(), v);
+    });
+
     m.def("cast_variant", []() {
         using V = std::variant<int, std::string>;
         return py::make_tuple(V(5), V("Hello"));

tests/test_python_types.py

@@ -373,12 +373,16 @@ def test_exp_optional():
 
 @pytest.mark.skipif(not hasattr(pybind11_tests, "load_variant"), reason='no <variant>')
 def test_variant(doc):
-    from pybind11_tests import load_variant, cast_variant
+    from pybind11_tests import load_variant, load_variant_2pass, cast_variant
 
     assert load_variant(1) == "int"
     assert load_variant("1") == "std::string"
     assert load_variant(1.0) == "double"
     assert load_variant(None) == "std::nullptr_t"
+
+    assert load_variant_2pass(1) == "int"
+    assert load_variant_2pass(1.0) == "double"
+
     assert cast_variant() == (5, "Hello")
 
     assert doc(load_variant) == "load_variant(arg0: Union[int, str, float, None]) -> str"